Now in order to better understand certain features of our invention, it may be well to note at this point that there are a great many alloys presently available for duties in the aerospace industries, the chemical industries, and others where strength, resistance to corrosion, ease of fabrication, and the like are required. And many of these are suited to applications where there are encountered high temperatures in use and where there additionally are encountered corrosive atmospheres--atmospheres containing chlorides and atmospheres of a general oxidizing character. We refer particularly to certain of the nickel-base alloys such as "Inconel 625" (20% chromium, 9% molybdenum, 3% columbium, 3% iron, and remainder nickel). These alloys, because of the high nickel content, are costly.
A further class of alloys is the chromium-nickel austenitic stainless steels, notably AISI Type 304 (18% to 20% chromium, 8% to 12% nickel, manganese 2% max., carbon 0.08% max., and remainder iron), Type 347 (17% to 19% chromium, 9% to 13% nickel, 2% max. manganese, carbon 0.08% max., with columbium-tantalum at least 10 times the carbon content, and remainder iron). A further and more important stainless steel is the Type 316 (16% to 18% chromium, 10% to 14% nickel, 2% to 3% molybdenum, 2% max. manganese, carbon .08% max., and remainder iron).
A still further stainless steel is the Armco 21-6-9 (20.25% chromium, 6.5% nickel, 9% manganese, .15% to .40% nitrogen, carbon not exceeding .08%, and remainder iron).
Although the austenitic chromium-nickel stainless steels referred to are less costly than the nickel-base alloys, they either are lacking in weldability, are characterized by a low ratio of strength-to-weight, or have insufficient strength to meet the requirements of many applications.
Accordingly, it is one of the objects of our invention to provide a chromium-nickel stainless steel which is significantly leaner than the nickel-base alloys, which chromium-nickel stainless steel is substantially austenitic under all conditions, which readily lends itself to conversion in the hot-mill and the cold-mill, which lends itself to a variety of forming and fabricating operations, which is readily weldable, and which in welded condition is resistant to intercrystalline corrosion and to the general corrosion conditions encountered in the aerospace, the chemical, and a variety of industries in which welded assemblies are required.